Anti-fouling ultrafiltration membrane prepared from polysulfone-graft-methyl acrylate copolymers by UV-induced grafting method.

Abstract

Membrane fouling is one of the most important challenges faced in membrane ultrafiltration operations. The copolymers of polysulfone-graft-methyl acrylate were synthesized by homogeneous photo-initiated graft copolymerization. The variables affecting the degree of grafting, such as the time of UV (Ultraviolet-visible) irradiation and the concentrations of the methyl acrylate and photoinitiator, were investigated. The graft copolymer membranes were prepared by the phase inversion method. The chemical and morphological changes were characterized by attenuated total reflection-Fourier transform infrared spectroscopy (ATR/FT-IR), scanning electron microscopy, and water contact angles measurements. Results revealed that methyl acrylate groups were present on the membranes and the graft degree of methyl acrylate had remarkable effect on the performance of membranes. Pure water contact angle on the membrane surface decreases with the increase of methyl acrylate graft degree, which indicated that the hydrophilicity of graft copolymer membranes was improved. The permeation fluxes of pure water and bovine serum albumin solution were measured to evaluate the antifouling property of graft copolymer membranes, the results of which have shown an enhancement of antifouling property for graft copolymer membranes.

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jesc.ac.cn
Journal of Environmental Sciences 20(2008) 565–570
Anti-fouling ultrafiltration membrane prepared from polysulfone-graft-methyl
acrylate copolymers by UV-induced grafting method
HUA Helin1,2, LI Na2, WU Linlin1, ZHONG Hui1, WU Guangxia1,∗,
YUAN Zonghuan3, LIN Xiangwei3, TANG Lianyi3
1. Research Center for Eco-Environmental Science, Chinese Academy of Sciences, Beijing 100085, China. E-mail: huahelin76@yahoo.com.cn
2. Department of Environment and Chemistry Engineering, Nanchang Hangkong University, Nanchang 330034, China
3. The Greatwall Pharmaceutical Factory of Beijing, Beijing 100071, China
Received 27 July 2007; revised 9 October 2007; accepted 14 November 2007
Abstract
Membrane fouling is one of the most important challenges faced in membrane ultrafiltration operations. The copolymers of
polysulfone-graft-methyl acrylate were synthesized by homogeneous photo-initiated graft copolymerization. The variables affecting
the degree of grafting, such as the time of UV (Ultraviolet-visible) irradiation and the concentrations of the methyl acrylate and
photoinitiator, were investigated. The graft copolymer membranes were prepared by the phase inversion method. The chemical
and morphological changes were characterized by attenuated total reflection-Fourier transform infrared spectroscopy (ATR/FT-IR),
scanning electron microscopy, and water contact angles measurements. Results revealed that methyl acrylate groups were present on
the membranes and the graft degree of methyl acrylate had remarkable effect on the performance of membranes. Pure water contact
angle on the membrane surface decreases with the increase of methyl acrylate graft degree, which indicated that the hydrophilicity of
graft copolymer membranes was improved. The permeation fluxes of pure water and bovine serum albumin solution were measured
to evaluate the antifouling property of graft copolymer membranes, the results of which have shown an enhancement of antifouling
property for graft copolymer membranes.
Key words: polysulfone; methyl acrylate; UV irradiation; graft polymerization
Introduction
Ultrafiltration membranes (UF) are used extensively in
water/wastewater treatment, reverse osmosis pretreatment,
and separations in the food, dairy, paper, textile, chemical,
and biotechnology industries (Baker, 2004). However, the
flux decline caused by concentration polarization and
membrane fouling by proteins and other biomolecules
adsorption in the feed stream is a repugnant challenge,
which to a great extent prevents the wide-scale application
in these industries (Marshall et al., 1993). Concentration
polarization and membrane fouling not only decrease
membrane permeability, but also shorten membrane life
due to the aggressive chemicals necessary for cleaning.
When cleaning becomes ineffective, the membranes must
be replaced. Concentration polarization decreases the driv-
ing force of water flow across the membrane due to a local
increase in foulant concentration. The effect is completely
reversible, and can be reduced by modifying the flow
over the membrane. Membrane fouling itself can occur
in two ways: cake formation and absorption of foulants
(Hilal et al., 2005). Cake fouling is generally reversible
by water flushing or backwashing. However, fouling due
* Corresponding author. E-mail: wgxzhlj@yahoo.com.cn.
to the adsorption of foulants is essentially irreversible and
can only be counteracted to a certain extent by aggressive
chemical cleaning. Foulant adsorption can occur both on
the membrane surface and in the pores. Since hydrophobic
adsorption of foulants on membrane surface plays a key
role in membrane fouling, hydrophilic modification of
polymeric membrane surface can be one of the fouling
mitigation methods (Pinnau et al., 2000). Many investiga-
tions have demonstrated that increasing membrane surface
hydrophilicity could effectively inhibit membrane fouling
in water/wastewater treatment. Therefore, various methods
includingblending,coating,adsorption,chemical-grafting,
and radiation-induced grafting have been invented to mod-
ify membrane surface using hydrophilic modifiers (Roux
et al., 2006; Meincken et al., 2005).
Polysulfone is very popular as an ultrafiltration mem-
brane material. However, because of its hydrophobic
nature, polysulfone membranes are susceptible to fouling
by the adsorption of foulants (Song et al., 2004). There-
fore, there are growing interests in developing hydrophilic
polysulfone membrane surfaces. Several methods have
been created to obtain hydrophilic polysulfone membrane
surfaces, such as hydrophilic polymer coatings deposited
onto the polysulfone membrane surface (Lee et al., 2000;

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566 HUA Helin et al.Vol. 20
Higuchi et al., 2003); chemical reaction of hydrophilic
components onto the bulk polysulfone backbone (Sum-
mers et al., 2003; Lu et al., 2005; Park et al., 2006);
hydrophilic layers graft-polymerized onto polysulfone
membranes using radical reactions generated with low
temperature plasma (Steen et al., 2001), gamma radiation
(Mok et al., 1994) or UV irradiation (Kaeselev et al., 2001;
Taniguchi et al., 2004).
A promising alternative approach to UF membrane sur-
face modification that avoids extra processing steps is the
addition of an amphiphilic component to the hydrophobic
bulk material that can selectively segregate to the surface
duringmembranecasting,producingahydrophilicsurface.
For example, Chen et al. (1996) blended sulfonated or am-
inated polysulfone with polysulfone to obtain membranes
with increased flux. Ishihara and coworkers (Ishihara et
al., 1999) prepared polysulfone membranes incorporating
phospholipidbased polymers and demonstrated reduced
protein adsorption and low platelet adhesion relative
to polysulfone. Kim and coworkers (Kim et al., 2005)
employed polysulfone/poly (ethylene oxide) block copoly-
mers as an additive in polysulfone UF membranes to
improve hydrophilicity and protein fouling resistance.
In this study, the amphiphilic graft copolymers of
polysulfone-graft-methyl acrylate were synthesized using
homogeneous photo-initiated graft polymerization. The
obtained graft copolymers with varying graft methyl
acrylate content were cast for UF membranes using
the phase inversion method. The graft methyl acrylate
content, wettablity, pure water flux and fouling resis-
tance of the membranes were investigated by attenuated
total reflection-Fourier transform infrared spectroscopy
(ATR/FT-IR),contactanglemeasurementanddead-endfil-
tration. Scanning electron microscopy (SEM) was further
performed to assess the influence of the degree of graft
on membrane morphology. The membranes of the graft
copolymers were then compared with the virgin polysul-
fone membrane for resistance to fouling by model protein
(bovine serum albumin, BSA) solutions using dead-end
filtration.
1 Materials and methods
1.1 Materials
Polysulfone (PSF, its specific viscosity is 0.73) was
purchased from Dalian Polysulfone Plastic Co. Lt., Chi-
na. Before use, it was dissolved in dichloromethane and
precipitated into hexane. This purification procedure was
repeated twice, and the final precipitate was dried under
vacuum at 120°C for 12 h. Dichloromethane (CH2Cl2),
methyl acrylate (MA), benzophenone (BP), ethyl ether and
N-Methyl-2-pyrrolidinone (NMP) were purchased from
Peking Chemistry and Reagent Co., China. Bovine serum
albumin (BSA) was from Dongfang Instrument and Equip-
ment Co., Chinese Academy of Sciences, China, and
stored in safe. MA was distilled in vacuum to remove the
polymerization inhibitor before use. Other reagents were
used as received.
1.2 Synthesis of PSF-graft-MA copolymers
PSF-graft-MA (PSF-g-MA) copolymers were per-
formed by irradiating homogeneous solutions of PSF, MA
monomer and BP photoinitiator with UV light (the emis-
sion maximum at 365 nm and the intensity 20 mW/cm2).
Predetermined amounts of PSF, MA and BP were dis-
solved in tetrahydrofuran (THF) and the concentrated
solutions so obtained were poured in a quartz vessel, and
then the quartz vessel was placed in a photochemical
reactor. To remove the dissolved oxygen from the solution,
nitrogen gas was bubbled into the solution at a flow rate of
200 ml/min for 10 min. The quartz vessel was irradiated
with UV light at room temperature. After a predetermined
time of graft copolymerization, the contents of the quartz
vessel were precipitated in excess ethyl ether with stirring.
The suspension obtained was filtered and then the solid
product was vacuum dried to constant weight. The product
was purified by dissolution in THF and precipitated in
distilled water. The precipitates were washed with hot
water to remove the homopolymer after filtration. This
purification procedure was repeated twice. Finally, the
product was vacuum dried at 80°C before analysis and
use. The degree of grafting (DG) was calculated from the
following equation:
DG(%) = (Wg− W0
W0
) × 100 (1)
where, W0is the weight of initial PSF and Wgis the weight
of graft copolymer after complete removal of unreacted
monomer and homopolyer.
1.3 Membrane casting
The membranes were prepared from 15% (W/W) poly-
mer (PSF or PSF-g-MA copolymer) solutions in NMP,
then filtering and degassing under vacuum. Solutions were
castonaglassplatetoformthinfilms.Afterexposuretoair
for 10 s, the thin films were immersed in distilled water at
25°C for 2 min. The obtained membrane was then placed
in a distilled water bath overnight and stored in distilled
water before use.
1.4 Membrane characterization
To investigate the chemical changes between the nascent
membrane and graft copolymer membranes, attenuated
total reflection-Fourier transform infrared (ATR/FT-IR)
spectra with a Nicolet Avatar 360 FT-IR Spectrometer
(Thermo Fisher Scientific Inc., USA) was used. Prior to
the measurements, the samples were dried under a vacuum
at 60°C for 24 h. Data were recorded between 800–2,000
cm−1with 32 scans.
The morphology of the membranes, cross-section and
surface were imaged with SEM (JSM-6301F, JEOL Ltd.,
Japan). Membranes were dried by Critical Point Dryer
(EM Technologies Ltd., England) and then sputtered with
gold. All SEM photographs were obtained under 5 keV at
20°C.
Contact angle measurements were performed on the
cast membranes at 20°C with a CA-D contact angle
meter (Kyowa Interface Science Co., Ltd., Japan) using
de-ionized water as probe liquid. The membranes were

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No. 5Anti-fouling ultrafiltration membrane prepared from polysulfone-graft-methyl acrylate copolymers ······
fixed flat on a glass slide using double-sided tape and
dried for 2 h in air and 2 h in a vacuum oven before
the measurements. Five measurements were immediately
taken after the droplet was placed on the membrane.
Membrane wet ability was studied by placing 1 µl of de-
ionized water on the membrane surface, observing on the
contact angle meter and measuring the time required for
the water to be absorbed completely into the membrane.
The values reported are averages of five measurements.
567
1.5 Filtration experiments
This process was conducted according to the report of
Chen and Belfort (1999). The transmembrane pressure was
kept constant at 0.1 MPa and the filtration cell was stirred
at 500 r/min using a stir plate to minimize concentration
polarization. In a typical run, the solution reservoir was
initially filled with de-ionized water, and the membrane
was precompacted for 30 min during the filtration of
de-ionized water. The permeation data were collected
at fixed time intervals and weighed to determine trans-
membrane flux, until the flux remained constant for at least
three successive readings. The average of five readings
was recorded as J0. Next, the reservoir was emptied and
refilled with the model foulant solution. Protein solutions
comprised 1000 mg/L BSA in a phosphate-buffer solution
(4.56 g NaH2PO4, 23.00 g Na2HPO4, 149.76 g NaCl, 4.02
g KCl, de-ionized water 1000 ml, pH 6.9). The flux of
BSA filtration (J1) was recorded until the flux did not
change any longer, which usually needed at least 40 min.
A sample of permeate was collected after 1 h of filtration.
BSA retention values were obtained by measuring the
foulant concentration in this sample using UV-visible
spectroscopy with a LengGuang 752 ultraviolet-visible
spectrophotometer (Shanghai Lengguang Technology Co.,
Ltd., China). The concentrations were quantified using UV
absorbance at 280 nm for BSA. After that, the filtration
cell was rinsed five to seven times with de-ionized water.
Then the membrane was removed and inverted. De-ionized
water was passed through the reversed membrane at 0.1
MPa pressure for 30 min. Finally, the membrane was
flipped back to its original orientation and the de-ionized
water flux measure was designed as J2.
2 Result and discussion
2.1 Synthesis of PSF-g-MA copolymers
UV-induced graft copolymerization is one of the most
widely used techniques with vinyl monomers to natural
and synthetic polymers. As we known, one of the most
effective factors is the UV irradiation time. Therefore, the
influence of UV irradiation time on the graft degree of MA
was examined and the results are shown in Fig.1. It can
be seen that the degree of graft (DG) of MA onto PSF
increased progressively with UV irradiation time up to 10
min, and then decreased. This type of DG behaviour is
generally observed in experiments carried out using photo-
induced graft copolymerization. This may be explained by
the fact that as the irradiation time increased, the number
of grafting sites on the backbone increased. As a result,
Fig. 1
concentration = 0.25 wt.%).
Effect of the UV irradiation time on the degree of grafting (BP
the extent of initiation and propagation of photografting
copolymerization also increased with the irradiation time.
However, beyond the maximum irradiation time, as the
number of availablesites for the photografting of monomer
on the PSF backbone decreased, the reduction of DG was
observed with the irradiation. Another possible explana-
tion for the decrease in the DG at longer irradiation time
is that the chain scissions of PSF are caused as a result
of direct absorption of the incident radiation by the basic
backbone structure of PSF.
The MA concentration is another key factor in re-
lating to the graft degree of MA of graft copolymers
and the typical results are listed in Table 1. The graft
degree increased with increasing the MA concentration
to a maximum value, and then leveled off. At a low
monomer concentration, MA tends to graft onto the PSF
backbone, causing the grafting percentage to increase at
a given rate. With a further increase of the monomer
concentration, the graft level dropped. This is because
as the amount of monomer increases, there is an excess
of monomer competing for the initiator, resulting in the
homopolymerization and crosslink of the monomer as well
as the shortage of available grafting sites.
2.2 Membrane characterization
The membrane surface was characterized by ATR/FT-IR
spectroscopy and the typical spectra are depicted in Fig.2.
The spectra of all membranes are similar at wavelengths
below 1600 cm−1. The 1578 and 1486 cm−1bands are
due to the aromatic groups, and the 1323 cm−1/1289 cm−1
doublet and 1151 cm−1band are assigned to the aromat-
ic sulfone chromophore. Compared the graft copolymer
membranes with the nascent one, a new peak appears at
Table 1
DG of PS-g-MA graft copolymer in various reaction
conditions
PolymerReaction conditions
UV irradia-
tion time (min)
DG
(wt.%)
PSF:MA
(mass ratio)
BP in reac-
tion (wt.%)
PSF-g-MA1
PSF-g-MA2
PSF-g-MA3
PSF-g-MA4
PSF-g-MA5
1:0.16
1:0.32
1:0.48
1:0.64
1:0.80
6
8
8
10
10
0.12
0.25
0.25
0.36
0.36
2.95
7.07
10.62
14.89
12.35
PS-g-MA: polysulfone-graft-methyl acrylate; DG: degree of graft; PSF:
polysulfone; MA: methyl acrylate; BP: benzophenone.

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568HUA Helin et al.Vol. 20
Fig. 2
ATR/FT-IR spectra of the membranes.
1740 cm−1, which is corresponding to the typical absorp-
tion of carbonyl group in the MA. This new peak confirms
the presence of MA on the surface of graft copolymer
membranes. Moreover, with the increase of MA graft
degree, the intensity of the peak becomes stronger.
Pure water fluxes of the nascent and graft copolymer
membranes with the different graft degree are list in Table
2. The nascent PSF membrane exhibits low pure-water
flux as compared with the graft copolymer membranes,
indicating that the introduction of polar groups such as
the carboxyl group does improve membrane permeability
properties. This is because introducing MA onto the PSF
backbone enhances the hydrophilicity of the membranes.
The angle of water is generally used to evaluate the
hydrophilicity of the membranes. As expected, all the
graft copolymer membranes (Table 2) have lower contact
angles than the nascent PSF membrane. With the increase
of MA graft degree, the contact angle value decreases
from 68◦to 51◦. Referring to the FT-IR data (Fig.2), it
can be inferred that the higher average degree of MA
grafting on PSF result in membranes more hydrophilic.
However, the smaller pure water flux values are found in
the graft copolymer membranes of PGMA3 and PGMA4
as compared with PGMA2. In general, the variations in
permeate flux data could be attributed to the degree of
grafting, surface roughness, pore structure, pore sizes and
pore size distribution. Thus, for PGMA2 membrane, the
pure water flux is the highest, most probably as a result of
larger pore sizes and pore density.
The SEM photographs reveal distinctly different mor-
phological patterns for each of the membranes produced.
The bottom view and the cross-sections of the membranes
are shown in the SEM-micrographs of Fig.3. General
structures of membranes are very similar, consisting of
a thin dense layer at the top of the membrane, with an
intermediate layer with a finger-like structure and porous
architecture at the bottom. Morphologies of the ungraft-
ed and grafted membranes are very similar, except that
the amount of finger-like structure of graft copolymer
membranes decreased and a larger finger-like type of
structure is observed below the skin layer. In addition,
the graft copolymer membranes with high graft degree
show a relatively suppressed macropore structure and a
much thicker sponge-like layer at the bottom of the cross
section. The SEM analysis of the bottom view of the
different membranes (Fig.3) shows that the photograph for
Fig. 3
SEM micrographs of the bottom view (b) and the cross-sections of the membranes (PSF, PGMA1, PGMA2, PGMA4).

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No. 5Anti-fouling ultrafiltration membrane prepared from polysulfone-graft-methyl acrylate copolymers ······
Table 2
Membrane compositions and performance characteristics
569
MembranePolymer
(in Table 1)
Pure water permeability
(L/(m2·h))
252
389
528
296
264
BSA retention
after 1 h (%)
Wetting
time (min)
Contact
angle (◦)
PSF
PGMA1
PGMA2
PGMA3
PGMA4
Neat PSF
PSF-g-MA1
PSF-g-MA2
PSF-g-MA3
PSF-g-MA4
97.9
96.2
92.4
98.1
98.5
26
19
17
16
14
68
63
60
58
51
the nascent PSF membrane is only a well defined smooth
area. In the graft copolymer membranes, the Swiss-cheese
morphology with smooth areas between pore openings is
found. In contrast, a bottom view of the PGMA2 mem-
brane shows more pronounced Swiss-cheese structures
with different sizes and shapes as well as variable pore
sizes between the polymer fragments.
2.3 Permeation and antifouling property of the mem-
branes
The ATR/FT-IR, wetting, and contact angle results
all revealed localization of PSF-g-MA to the membrane
surfaces. To determine the effect on fouling resistance,
the protein filtration was performed on the neat PSF,
PGMA1, PGMA2, PGMA3 and PGMA4 with a feed
solution containing 1000 mg/L BSA in the phosphate-
buffer solution. The results of these filtration runs as a
function of the filtrate time are shown in Fig.4. As seen in
the data, the flux decline behaviors during protein filtration
were quite similar for all membranes. After 1 h filtration,
the flux was reduced to 30%–50% of initial flux for all
the membranes investigated, with higher flux declines
Fig. 4
branes during the BSA filtration.
Flux decline behavior of neat PSF and graft copolymer mem-
observed for membranes having higher initial flux. Fur-
thermore,nascentPSFmembraneshowedagreateramount
of flux decline compared to graft copolymer membranes.
It is well known that membrane fouling can be influ-
enced by hydrodynamic conditions, such as permeation
drag and back transport, and chemical interaction between
foulants and membranes. Since all the membranes were
test at the same hydrodynamic condition, the different
fouling behavior means that surface property of membrane
was changed by MA component employment. Surface
of graft copolymer membranes can be more hydrophilic
than that of neat PSF membrane due to the hydrophilic
MA component employed into the bulk PSF backbone.
Therefore, hydrophobic adsorption between foulants and
graft copolymer membranes was reduced, and deposited
particles were readily removed by crossflow.
The permeation fluxes and flux changes during filtration
for nascent PSF and graft copolymer membranes with
different MA graft degree are shown in Fig.5. It can be
seen in Fig.5a that almost all graft copolymer membranes
experience both increase in the pure water and protein
solution fluxes. When filtrated with a 1000 mg/L BSA so-
lution, the membranes has an enhancing tendency toward
antifouling property as the graft degree of MA increases,
which is shown by the decrease of the total flux loss (1–
J0/J1), from 79% for the neat membrane to 47% for the
PGMA4 membrane grafted with 14.89 wt.% MA (Fig.5b).
The most notable differences between different polymer
membranes were observed in the recovery step after a de-
ionized water rinse. Cleaning the membrane was intended
to remove BSA molecules adsorbed on the membrane sur-
face, and the results of washing demonstrated the recovery
ability of membrane fouled by protein. From the columns
shown in Fig.5b, it can be seen that graft copolymer
membranes are much more easily recovered with washing,
and the overall water flux recovery (J2− J1)/(J0− J1) is
Fig. 5 Permeation fluxes (a) and flux changes (b) during filtration for neat PSF membrane and graft copolymer membranes with different MA graft
degree.